Synthesis, characterization, and lead removal efficiency of orange peel powder and orange peel powder doped iron (III) oxide-hydroxide

Lead contamination in wastewater causes toxicity to aquatic life, the environment, and water quality, and it causes many human dysfunctions and diseases. Thus, it is necessary to remove lead from wastewater before discharging it into the environment. Orange peel powder (OP) and orange peel powder doped iron (III) oxide-hydroxide (OPF) were synthesized, characterized, and investigated lead removal efficiencies by batch experiments, adsorption isotherms, kinetics, and desorption experiments. The specific surface area of OP and OPF were 0.431 and 0.896 m2/g, and their pore sizes were 4.462 and 2.575 nm, respectively which OPF had a higher surface area than OP, whereas its pore size was smaller than OP. They were semi-crystalline structures that presented the specific cellulose peaks, and OPF also detected the specific iron (III) oxide-hydroxide peaks. The surface morphologies of OP and OPF were irregular and porous surfaces. Carbon (C), oxygen (O), calcium (Ca), O–H, C–H, C=C, C–O, C=O, and –COOH were observed in both materials. The pHpzc of OP and OPF were 3.74 and 4.46. For batch experiments, OPF demonstrated a higher lead removal efficiency than OP because of spending less on material dosage than OP, and OPF demonstrated high lead removal by more than 95% while OP could remove lead at only 67%. Thus, the addition of iron (III) oxide-hydroxide helped to increase material efficiency for lead adsorption. Both materials corresponded to the Freundlich model relating to physiochemical adsorption, and they also corresponded to a pseudo-second-order kinetic model relating to a chemisorption process. Moreover, both materials could be reusable for more than 5 cycles for lead adsorption of more than 55%. Therefore, OPF was potential material to apply for lead removals in industrial applications.

. The elimination of heavy metals in wastewater from various waste peels.
The material synthesis of OP. Firstly, orange peels were washed with tap water to eliminate contaminations. Next, 40 g of orange peels were soaked in the solution of 95% C 2 H 5 OH and 0.5 M NaOH in a ratio of 2:1 for 12 h. Then, they were washed with distilled water until the solution turned to a pH of 7. After that, they were dried in Table 2. Several modification methods for improving the adsorbent capacity of various waste peels to eliminate heavy metals in wastewater.
The material synthesis of OPF. Firstly, 10 g of OP were added to 500 mL of Erlenmeyer flask containing 160 mL of 5% FeCl 3 ·6H 2 O. Then, they were mixed by an orbital shaker (GFL, 3020, Germany) of 200 rpm for 3 h. Next, they were filtrated and air-dried at room temperature for 12 h. After that, they were added to 500 mL of Erlenmeyer flask containing 160 mL of 5% NaOH, and they were mixed by an orbital shaker of 200 rpm for 1 h. Finally, they were filtered, air-dried at room temperature for 12 h, and kept in a desiccator before use called orange peel powder doped iron (III) oxide-hydroxide (OPF).    (1,3,5,7,9,11), and concentration (10 to 70 mg/L) with the control condition was the initial lead concentration of 50 mg/L, a sample volume of 100 mL, pH 5, a shaking speed of 200 rpm, and a temperature of 25 ℃. The optimum value was selected from the lowest value of each factor with the highest lead removal efficiency, and that value was applied to the next affecting factor study. Lead concentrations are measured by the Atomic Adsorption Spectrophotometer (AAS) (PerkinElmer, PinAAcle 900 F, USA), and triplicate experiments were conducted to confirm the results. Lead removal efficiency in the percentage (%) is calculated by following Eq. (1) where C 0 is the initial lead concentration (mg/L), and C e is the equilibrium of lead concentration in the solution (mg/L).

Adsorption isotherms.
Adsorption isotherms are designed to investigate the adsorption patterns of OP and OPF by using various adsorption models of linear and nonlinear Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich. Graphs of linear Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherms were plotted by C e /q e versus C e , log q e versus log C e , q e versus ln C e , and ln q e versus ε 2 , respectively whereas graphs of their nonlinear were plotted by q e versus C e . Their adsorption models are calculated by Eqs.
(2)-(9) 50-53 : Langmuir isotherm: Freundlich isotherm: Temkin isotherm: Dubinin-Radushkevich isotherm: where C e is the equilibrium of lead concentration (mg/L), q e is the amount of adsorbed lead on OP or OPF (mg/g), q m is indicated the maximum amount of lead adsorption on OP or OPF (mg/g), K L is the adsorption constant (L/mg). K F is the constant of adsorption capacity (mg/g) (L/mg) 1/n , and 1/n is the constant depicting the adsorption intensity. R is the universal gas constant (8.314 J/mol K), T is the absolute temperature (K), b T is the constant related to the heat of adsorption (J/mol), and A T is the equilibrium binding constant corresponding to the maximum binding energy (L/g). q m is the theoretical saturation adsorption capacity (mg/g), K DR is the activity coefficient related to mean adsorption energy (mol 2 /J 2 ), and ε is the Polanyi potential (J/mol). For adsorption isotherm experiments, 4 g of OP or 3 g of OPF were added to 250 mL Erlenmeyer flasks with variable lead concentrations from 10 to 70 mg/L. The control condition of OP or OPF was a sample volume of 100 mL, a shaking speed of 200 rpm, pH 5, a temperature of 25 °C, and a contact time of 6 h. Adsorption kinetics. Adsorption kinetics are used for studying adsorption rates and mechanisms of OP and OPF which various kinetic models of linear and nonlinear pseudo-first-order kinetic, pseudo-second-order  www.nature.com/scientificreports/ kinetic, elovich, and intraparticle diffusion were applied. Graphs of linear pseudo-first-order, pseudo-secondorder, elovich, and intraparticle diffusion models were plotted by ln (q e − q t ) versus time (t), t/q t versus time (t), q t versus ln t, and q t versus time (t 0.5 ), respectively whereas their nonlinear graphs were plotted by the capacity of lead adsorbed by adsorbent materials at the time (q t ) versus time (t). Their adsorption kinetic equations are calculated by Eqs. (10)-(16) 54-57 : Pseudo-first-order kinetic model: Pseudo-second-order kinetic model: Elovich model: where q e is the amount of adsorbed lead on adsorbent materials (mg/g), q t is the amount of adsorbed lead at the time (t) (mg/g), k 1 is a pseudo-first-order rate constant (min −1 ), and k 2 is a pseudo-second-order rate constant (g/mg•min). α is the initial adsorption rate (mg/g/min) and β is the extent of surface coverage (g/mg). k i is the intraparticle diffusion rate constant (mg/g•min 0.5 ) and C i is the constant that gives an idea about the thickness of the boundary layer (mg/g). For adsorption kinetic experiments, 40 g of OP or 30 g of OPF were added to 1000 mL of breaker with the lead concentration of 50 mg/L. The control condition of OP and OPF was a sample volume of 1000 mL, a shaking speed of 200 rpm, pH 5, a temperature of 25 °C, and a contact time of 8 h.
Desorption experiments. The five adsorption-desorption cycles are designed for desorption experiments of OP and OPF for lead adsorption to investigate the possible material reusability. The saturated OP or OPF from the adsorption process was added to 500 mL of Erlenmeyer flask containing 200 mL of 0.5 M HNO 3 solution, and it was shaken by an incubator shaker (New Brunswick, Innova 42, USA) at 200 rpm for 6 h. After that, it was washed with deionization water and dried at room temperature. Then, OP or OPF is ready for the next adsorption cycle. The desorption efficiency in percentage is calculated by following Eq. (17): where q d is the amount of lead desorbed (mg/mL) and q a is the amount of lead adsorbed (mg/mL).

Result and discussion
The physical characteristics of OP and OPF. The physical characteristics of OP and OPF are demonstrated in Fig. 2a,b. OP was a yellow color powder shown in Fig. 2a while OPF was an iron-rust color powder corresponding to a color of iron (III) oxide-hydroxide color shown in Fig. 2b.

Material characterizations of OP and OPF. BET. Brunauer-Emmett-Teller (BET) technique with N 2
adsorption-desorption isotherm at 77.3 K and degas temperature of 80 °C for 6 h was used for determining the specific surface area, pore volume, and pore diameter size of OP and OPF. The results of the surface area and pore volume are reported by Brunauer-Emmett-Teller (BET) method, whereas the pore size is reported by Barrett-Joyner-Halenda (BJH) method shown in Table 3. For OP, its specific surface, pore volume, and pore size were 0.431 m 2 /g, 0.099 cm 3 /g, and 4.462 nm, respectively. The specific surface, pore volume, and pore size of OPF were 0.896 m 2 /g, 0.206 cm 3 /g, and 2.575 nm, respectively. As a result, OPF had a higher specific surface area and pore volume than OP, whereas OPF had a smaller pore size than OP. Thus, the addition of iron (III) oxide-hydroxide into OPF affected to increase its specific surface area and pore volume and decrease its pore size similar reported by previous studies 6, [10][11][12] . Since the pore sizes of OP and OPF were in a range of 2-50 nm, they were classified to be as mesoporous material following the classification by the International Union of Pure and Applied Chemistry (IUPAC) 58 .
XRD. X-Ray Diffractometer (XRD) was used for characterizing the crystalline structures of OP and OPF, and their results are shown in Fig. 3a,b. OP and OPF were semi-crystalline structures that presented the specific cellulose peaks at 2θ values of approximately 15.57°, 22.71°, and 35.01°5 9 . In addition, the specific iron (III) www.nature.com/scientificreports/      www.nature.com/scientificreports/ and porous surfaces. OPF had the same structure and morphology surface as the OP, but its size was larger than the OP at the same magnification.
EDX. Energy Dispersive X-Ray Spectrometer (EDX) was used for determining the chemical compositions of OP and OPF, and their results are demonstrated in Table 4. Carbon (C), oxygen (O), and calcium (Ca) were the main chemical elements in both materials similar found in other studies 31,40 , whereas iron (Fe), sodium (Na), and chloride (Cl) were only found in OPF. Fe, Na, and Cl might be from using chemicals of ferric chloride hexahydrate (FeCl 3 ·6H 2 O) and sodium hydroxide (NaOH) for synthesizing OPF.
FT-IR. Fourier Transform Infrared Spectroscopy (FT-IR) was used for identifying the chemical functional groups of OP and OPF, and their FT-IR spectra are demonstrated in Fig. 5a Fig. 5b. As a result, the addition of iron (III) oxide-hydroxide affected to the higher stretching of all main functional groups of OPF more than OP which they might result to increase lead adsorption of OPF more than OP.
The point of zero charges (pH pzc ) of OP and OPF. The point of zero charge (pH pzc ) refers to a pH value at the net charge equal to zero of the adsorbent which uses for determining which pH value is good for lead adsorption by material. In Fig. 6, the pH pzc of OP and OPF were 3.74 and 4.46, so the addition of iron (III) oxide-hydroxide affected the increase of pH pzc similar reported by previous studies [9][10][11] . The pH of the solution and the pH pzc are normally used for considering lead adsorption by material which the high lead adsorption should be found at the pH of solution higher than pH pzc (pH solution > pH pzc ) because of occurring negatively charged of material. In addition, many previous studies reported high lead adsorption at a pH solution higher than pH 4 5 Fig. 7d. Lead removal efficiencies of both materials were decreased with the increasing of concentrations which might be from the active sites of them did not enough to caught up with lead ions similarly reported by other studies 5,8,64 . For the lead concentration of 50 mg/L, lead removal efficiencies of OP and OPF were 69.78% and 95.29%, and OPF demonstrated a higher lead removal efficiency than OP.
In conclusion, 4 g, 6 h, pH 5, 50 mg/L and 3 g, 6 h, pH 5, 50 mg/L were the optimum conditions in dose, contact time, pH, and concentration of OP and OPF. As a result, adding iron (III) oxide-hydroxide helped to improve material efficiency for lead adsorption similar reported by previous studies 5, 6 , and OPF was recommended to be applied for lead removal in future industrial applications.
Adsorption isotherms. The adsorption patterns of OP and OPF for lead adsorptions were investigated through linear and nonlinear models of Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models. For linear models, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherms were plotted by C e /q e versus C e , log q e versus log C e , q e versus ln C e , and ln q e versus ε 2 , respectively. For nonlinear models, all isotherms were plotted by C e versus q e. The plotting graphs are demonstrated in Fig. 8a-f, and the equilibrium isotherm parameters are illustrated in Table 5.  For R 2 value consideration, since R 2 values of OP and OPF in both linear and nonlinear Freundlich models were higher than Langmuir, Temkin, and Dubinin-Radushkevich models, their adsorption patterns corresponded to Freundlich isotherm relating to physiochemical adsorption. Therefore, it recommends plotting isotherm graphs in both linear and nonlinear models to confirm the results and protect against data mistranslation [65][66][67] .
Moreover, the comparison of the maximum adsorption capacity (q m ) value of waste peel adsorbents for lead adsorption is illustrated in Table 6. The orange peel powder doped iron (III) oxide-hydroxide (OPF) demonstrated www.nature.com/scientificreports/ a higher q m value than the pomelo, banana, and lemon peels 5,18,68 whereas the orange peel powder (OP) was a lower q m value than all studies. Therefore, the addition of iron (III) oxide-hydroxide into orange peel powder in this study helped to increase the maximum adsorption capacity of orange peel material. In addition, the raw material plays the main role in lead adsorption resulting in different lead adsorption capacities. Furthermore, comparing lead adsorption by orange peels of this study and the study of Chinyelu et al. found that the material size might be another effect to lead adsorption which the smaller material size could highly remove lead. However, the costs of material synthesis and material separation after the treatment of small materials are higher than big materials, so the operation cost might be a concern for real applications.
Adsorption kinetics. The adsorption mechanism and reaction rate of OP and OPF for lead adsorptions were investigated by linear and nonlinear kinetic models of a pseudo-first-order kinetic model, pseudo-second-order kinetic model, elovich model, and intraparticle diffusion. For linear models, they were plotted by ln (q e − q t ) versus time (t), t/q t versus time (t), q t versus ln t, and q t versus time (t 0.5 ) for a pseudo-first-order kinetic, pseudo-second-order kinetic, elovich, and intraparticle diffusion models, respectively. For nonlinear models, they were plotted by q t versus time (t). The plotting graph results are illustrated in Fig. 9a-f, and the adsorption kinetic parameters are presented in Table 7.
For linear models, the adsorption capacities (q e ) of OP and OPF on a pseudo-first-order kinetic model were 0.988 and 1.091 mg/g, and their reaction of rate constants (k 1 ) were 0.012 and 0.013 min −1 . For a pseudosecond-order kinetic model, the adsorption capacities (q e ) of OP and OPF were 1.044 and 1.853 mg/g, and their reaction of rate constants (k 2 ) were 0.026 and 0.028 g/mg·min. For the elovich model, the initial adsorption rates (α) of OP and OPF were 0.748 and 0.935 mg/g/min, and their extents of surface coverage (β) were 6.053 and 3.560 g/mg. For the intraparticle diffusion model, the reaction of rate constants (k i ) of OP and OPF were 0.038 and 0.062 mg/g·min 0.5 , and their constant C i values were 0.230 and 0.540 mg/g. R 2 values of OP and OPF on pseudo-first-order and pseudo-second-order kinetic models were 0.975, 0.985 and 0.991, 0.994, respectively. In addition, R 2 values of OP and OPF on elovich and intraparticle diffusion models were 0.941, 0.968 and 0.803, 0.772, respectively.
For nonlinear models, the adsorption capacities (q e ) of OP and OPF on a pseudo-first-order kinetic model were 0.994 and 1.039 mg/g, and their reaction of rate constants (k 1 ) were 0.014 and 0.018 min −1 . For a pseudosecond-order kinetic model, the adsorption capacities (q e ) of OP and OPF were 1.050 and 1.872 mg/g, and their reaction of rate constants (k 2 ) were 0.023 and 0.029 g/mg·min. For the elovich model, the initial adsorption rates (α) of OP and OPF were 0.752 and 0.950 mg/g/min, and their extents of surface coverage (β) were 6.132 and 3.675 g/mg. For the intraparticle diffusion model, the reaction of rate constants (k i ) of OP and OPF were 0.041 and 0.067 mg/g·min 0.5 , and their constant C i values were 0.232 and 0.553 mg/g. R 2 values of OP and OPF on pseudo-first-order and pseudo-second-order kinetic models were 0.976, 0.988 and 0.992, 0.993, respectively. In addition, R 2 values of OP and OPF on elovich and intraparticle diffusion models were 0.943, 0.970 and 0.807, 0.774, respectively. Moreover, R 2 adj of OP and OPF in nonlinear pseudo-first-order and pseudo-second-order kinetic models were 0.974, 0.987 and 0.991, 0.992, respectively. R 2 adj of OP and OPF in nonlinear elovich and intraparticle diffusion models were 0.941, 0.968 and 0.805 0.772, respectively.
For R 2 value consideration, since R 2 values of OP and OPF in both linear and nonlinear pseudo-second-order kinetic models were higher than pseudo-first-order kinetic, elovich, and intraparticle diffusion models, so their adsorption rate and mechanism of both materials corresponded to pseudo-second-order kinetic model with relating to a chemisorption process with heterogeneous adsorption. Moreover, it also recommends plotting kinetic graphs in both linear and nonlinear models for confirming results and protecting against data mistranslations 69-72 . Desorption experiments. The possible reuses of OP and OPF are important points to estimate the cost and economic feasibility of industrial applications which were studied through the desorption experiments. The lead adsorption-desorption in 5 cycles is designed to investigate their reusable abilities, and their results are illustrated in Fig. 10a,b. OP could be reused in 5 cycles with high adsorption and desorption in ranges of 55.14- Table 6. Comparison of the maximum adsorption capacity (q m ) of various waste peels for lead adsorption.  Fig. 10a. OPF also confirmed to be reusability in 5 cycles with high adsorption and desorption in ranges of 86.57-96.64% and 80.20-94.23%, respectively which adsorption and desorption were decreased by approximately 10% and 14%, respectively shown in Fig. 10b. Therefore, both materials are potential materials for lead adsorption with the reusability of more than 5 cycles by more than 55%, and they can be further applied to industrial applications.  www.nature.com/scientificreports/

The possible mechanisms of lead adsorptions by OP and OPF
The possible mechanisms of lead adsorptions on OP and OPF are demonstrated in Fig. 11a,b which are modified an idea from the previous studies 6,9,10 . The main structures of OP and OPF are composed of cellulose, hemicellulose, pectin, and lignin including the main functional groups of the hydroxyl group (-OH). Since iron (III) oxide-hydroxide was added into OP to be OPF, the complex compound of OP•Fe(OH) 3 was found on the surface from adding iron (III) oxide-hydroxide into OP by sharing electrons with -OH of OP. The possible mechanism of lead adsorptions by OP and OPF might occur from donating a proton (H + ) from -OH or OP•Fe(OH) 3 of the main chemical compounds for capturing lead (II) ions (Pb 2+ ) instead of H + from a process of electrostatic interaction 6 .

Conclusion
Orange peel powder (OP) and orange peel powder doped iron (III) oxide-hydroxide (OPF) were successfully synthesized for lead adsorption in an aqueous solution. The specific surface area and pore volume of OPF were higher than OP, whereas its pore size was smaller than OP. They were semi-crystalline structures that presented the specific cellulose peaks, and OPF also detected the specific iron (III) oxide-hydroxide peaks. The surface morphologies of OP and OPF were irregular and porous surfaces. Three main chemical compositions of OP and www.nature.com/scientificreports/ OPF were carbon (C), oxygen (O), and calcium (Ca), whereas iron (Fe), sodium (Na), and chloride (Cl) were only detected in OPF with adding iron (III) oxide-hydroxide. Six main chemical functional groups of O-H, C-H, C=C, C-O, C=O, and -COOH were detected in both materials whereas Fe-O was only found in OPF. The pH pzc of OP and OPF were 3.74 and 4.46. For batch experiments, the optimum conditions of OP and OPF were 4 g, 6 h, pH 5, 50 mg/L and 3 g, 6 h, pH 5, 50 mg/L, and their lead removal efficiencies were 69.78% and 95.29%. As a result, OPF demonstrated a higher lead removal efficiency than OP because it spent less material dosage and gave a high percentage of lead removal than OP. Therefore, adding iron (III) oxide-hydroxide helped to improve orange peel efficiency for lead adsorption. For the isotherm study, both materials corresponded to the Freundlich model correlated to a physicochemical process. For the kinetic study, they corresponded to a pseudo-second-order kinetic model related to a chemisorption process with heterogeneous adsorption. Moreover, both materials could be reusable for more than 5 cycles for lead adsorptions of more than 55%. Therefore, OP and OPF were high-potential materials for lead adsorptions in an aqueous solution, and OPF demonstrated the highest lead removal efficiency. Therefore, OPF was suitable to apply for industrial wastewater treatment applications in the future.
In future works, the continuous flow study also needs to study for further industrial applications, and the competing ions such as sodium (Na + ), magnesium (Mg 2+ ), and natural organic matter (NOM) contaminated in real wastewater should be investigated to confirm the specific lead adsorption by OP or OPF. www.nature.com/scientificreports/

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.  www.nature.com/scientificreports/